The present invention generally relates to a recording apparatus for recording on an endless-belt-shaped recording material which is supported and driven by a plurality of rollers, in particular to a skew control apparatus for an endless-belt-shaped recording material for use in the recording apparatus.
In a recording apparatus of the above-mentioned type, an electrostatic recording type apparatus, a magnetic recording type apparatus and an electrophotographic recording type apparatus are known. In these apparatuses, it is important that such endless-belt-shaped recording material is always transported in the same posture.
The outline of a conventional recording apparatus of an electrophotographic type will now be explained by referring to FIG. 1.
FIG. 1 shows the main portion of the electrophotographic recording apparatus, in which, as the recording material, an endless-belt-shaped photoconductor 1 (hereinafter referred to as the photoconductor 1) comprising a base film made of polyethylene terephthalate and an organic or inorganic photoconductor deposited on the base film is employed. The photoconductor 1 is supported and driven in rotation in the direction of arrow A by a drive roller 2 and a driven roller 3. Around the photoconductor 1, there are arranged a charger 4 for applying charges to the photoconductor 1; an exposure apparatus 5 for exposing the electrically charged photoconductor 1 to a light image L of an original, thereby forming a latent electrostatic image on the photoconductor 1; a development apparatus 6 for developing the latent electrostatic image to a visible image by a developer; an image transfer charger 7 for transferring the visible image from the photoconductor 1 to a recording sheet; a cleaner 8 for cleaning the surface of the photoconductor 1 after the image development process; a quenching charger 9 for eliminating remaining charges from the surface of the photoconductor 1 in preparation for reuse of the photoconductor 1, followed by charging and exposure; and other members.
In a housing (not shown) of the recording apparatus, there is disposed a sheet stacking box (not shown), from which recording sheets are supplied and then discharged out of the recording apparatus through a path shown by the arrow P. In the course of the transportation, a visible image formed on the photoconductor 1 is transferred therefrom to a recording sheet by the image transfer charger 7.
The above-described recording apparatus employing the endless-belt-shaped photoconductor has an advantage over a recording apparatus employing a drum-shaped photoconductor in that the apparatus can be made small in size . On the other hand, the former has a shortcoming in that the belt-shaped photoconductor is apt to be skewed in the direction normal to the running direction of the belt. For instance, in the recording apparatus as shown in FIG. 1, the skewing of the photoconductor 1 occurs due to imperfections in the shapes, sizes and attachment positions of the drive roller 2 and driven roller 3 or due to the difference in tension applied to the photoconductor 1 by those rollers between the opposite ends thereof.
In case no countermeasures are taken against such skewing, the photoconductor 1 will gradually deviate from the correct position for latent image formation and visible image transfer.
In order to prevent such inconvenience, conventionally the skewing of the photoconductor 1 is prevented by disposing flanges at the opposite end portions of the shaft of the drive roller 2 or of the driven roller 3. This method is effective when the skewing of the photoconductor 1 is slight, since, in that case, the skewing is stopped by the side portions of the photoconductor 1 coming into contact with the flanges. However, when the variations in shape and size of the above-mentioned rollers are great and accordingly when the skewing force applied to the photoconductor 1 is great, the skewing cannot be stopped even if the side portions of the photoconductor 1 come into contact with those flanges. The result is that the side portions of the photoconductor 1 are damaged by the flanges and the image formation area in the photoconductor 1 undulates. As a matter of course, when this occurs, copy images faithful to the original images cannot be obtained.
The inventors of the present invention previously proposed a skew control apparatus capable of eliminating the above-described conventional shortcomings, which comprises an inclination mechanism for reversing the skewing direction of the endless-belt-shaped recording material by inclining one of the two rollers (corresponding to the rollers 2 and 3 in FIG. 1), over which the endless-belt-shaped recording material is trained, relative to the other roller, within a plane normal to the running direction of the recording material or within a horizontal plane; a skew detection means for generating a skewing detection signal upon detecting a predetermined amount of skewing of the recording material; and a drive switching means for receiving the detection signal generated from the skew detection means and switching the position of the inclination mechanism to its operational position as long as the detection signal is generated.
By this skew control apparatus, the shortcomings of the conventional skew control apparatus, such as the damage to the side portions of the belt-shaped recording material and the undulation of the recording material caused by its skewing, can be eliminated, since the skew detection means and the inclination mechanism are constructed in such a manner as not to apply pressure to the side edge portions of the belt-shaped recording material.
In the above-described skew control apparatus, upon detecting the skew, the skew-reversing control is done by switching the position of one or the other of the rollers to a first inclined position or a parallel position to a second oppositely inclined position. The skew-reversing control is instantly performed by a magnetic solenoid or the like, and the skewing is gradually corrected as the belt runs. As a matter of course, such skew-reversing control, that is, switching the inclined position of each roller, may be done during the recording operation of the recording apparatus. When it is done during the recording operation, the image quality may vary before and after such switching of the position of the rollers. Referring to FIG. 1, for instance, when the roller 3 is inclined during the operation of the image transfer charger 7, the gap between the photoconductor 1 and the image transfer charger 7 is abruptly changed, so that the image transfer efficiency changes before and after the inclination of the roller 3. The result is that the transferred image may be blurred. Likewise, since it is considered that the peripheral speed of the photoconductor 1 may be temporarily changed when the roller 3 is inclined, it is preferable that the roller 3 not be inclined during the exposure operation by the exposure apparatus 5. In particular, when the exposure is done by laser beams, images may be blurred considerably. This is because, in the case of the exposure by laser beams, the recording material is moved in the sub-scanning direction by an extremely small distance, for instance, a certain fraction of 1 mm, after each line of the main scanning. Therefore, so long as the load applied to the drive system of the belt is changed, regardless of the switching direction of the skewing, the peripheral speed of the photoconductor is temporarily changed. The result is that the images are considerably blurred at the time of exposure.